1. Field of the Invention
The present invention relates to silicon carbide (SiC) single crystals and, more particularly, to a method of fabricating SiC single crystals using a gaseous source of deep level dopant.
2. Description of Related Art
Single crystals of silicon carbide of 4H and 6H polytypes serve as lattice-matched substrates in SiC- and AlGaN-based semiconductor devices, including ultra-high-frequency AlGaN-based transistors and SiC-based devices for power switching. Other applications include ultra-fast photoconductive switches, sensors for harsh environments, radiation detectors and many others. In the cases of high-frequency devices and photoconductive switches, the SiC substrates must be semi-insulating, that is having very high electric resistivity.
In the past, the term ‘semi-insulating’ in application to SiC meant simply that the crystal resistivity was above 1·105 Ohm-cm. In more stringent terms, ‘semi-insulating’ is a synonym for ‘fully compensated’. Many of the present day semiconductor devices built on SiC substrates require the substrate to have a resistivity on the order of 1010-1011 Ohm-cm or higher.
Compensation of silicon carbide with vanadium is well known and has been used to produce SiC single crystals having high electric resistivity. The Prior Art related to vanadium doping includes U.S. Pat. Nos. 5,611,955; 7,608,524; 8,216,369; US 2008/0190355 and US 2011/0303884, which are all incorporated herein by reference. Vanadium produces two deep levels in the SiC bandgap, one deep acceptor and one deep donor, and, therefore, can electronically compensate either shallow donors (when they dominate over shallow acceptors), or shallow acceptors (when they dominate over shallow donors).
Large-size commercial SiC single crystals are commonly grown by the sublimation technique of Physical Vapor Transport (PVT). A simplified schematic diagram of conventional PVT system is shown in FIG. 1, wherein a double-wall, water-cooled furnace chamber 1 is desirably made of fused silica. A PVT crystal growth cell is disposed inside furnace chamber 1. The PVT growth cell includes crystal growth crucible 20 charged with polycrystalline SiC grain 21 (SiC source) and a SiC single crystal seed 22 in spaced relationship. Commonly, SiC source 21 is disposed at the bottom of growth crucible 20, while SiC seed 22 is disposed at the top of growth crucible 20. Desirably, growth crucible 20 is made of dense, fine-grain graphite.
Conventionally, inductive type of heating is employed in PVT growth of silicon carbide. This type of heating is shown in FIG. 1 by RF coil 11 which is disposed outside the chamber 1. Graphite growth crucible 20 serves as an RF susceptor which couples electromagnetically to an RF field produced by excitation of RF coil 11. Growth crucible 20 is surrounded by thermal insulation 10 which is usually made of light-weight porous graphite, such as graphite felt or fibrous graphite board. These thermally insulating materials do not couple substantially to the RF field of RF coil 11. Resistive-type heating (in place of or in addition to RF coil 11), different types of thermal insulation, furnace chambers made of stainless steel, and RF coils disposed inside the chamber can also or alternatively be successfully employed for SiC sublimation growth. Other common and ordinary parts of the PVT crystal growth apparatus, such as gas and vacuum lines, valves, pumps, electronic controls, etc. are not shown in FIG. 1.
In preparation for PVT growth, chamber 1 is loaded with growth crucible 20 charged with SiC source 21 and SiC seed 22, and thermal insulation 10. Chamber 1 and, hence, growth crucible 20 are then evacuated and filled with a process gas (most commonly argon) to a desired pressure—generally between several and 100 Torr. Following this, growth crucible 20 is heated via energized RF coil 11 to growth temperature, which is generally between 2000° C. and 2400° C. Growth crucible 20 is heated such that a vertical temperature gradient is created between SiC source 21 and SiC seed 22, with the temperature of SiC source 21 higher than that of SiC seed 22.
At high temperatures, SiC source 21 sublimes releasing into the atmosphere of growth crucible 20 a spectrum of volatile molecular species, such as Si, Si2C and SiC2. Driven by the vertical temperature gradient, these species migrate to SiC seed 22 (vapor transport in FIG. 1 is shown by arrow 23) and condense on it causing growth of SiC single crystal 24 on SiC seed 22. Prior art in the area of PVT growth of silicon carbide includes U.S. Pat. Nos. 6,805,745; 5,683,507; 5,667,587 and 5,746,827, which are all incorporated herein by reference.
In the past, vanadium-doped SiC crystals (such as SiC crystal 24) were obtained by admixing a small amount of solid vanadium dopant directly to the SiC source (such as SiC source 21), as disclosed in U.S. Pat. No. 5,611,955 and US 2008/0190355, both of which are incorporated herein by reference. This solid vanadium dopant could be in the form of elemental metallic vanadium or in the form of a solid vanadium compound such as vanadium carbide. A major disadvantage of this type of vanadium doping is the physical contact between the solid vanadium dopant and the SiC source 21. Specifically, at high temperatures, multi-step chemical reactions take place between the vanadium and the SiC source 21 leading to the formation of multiple intermediary compounds, such as vanadium carbides, silicides, carbo-silicides and various eutectic compositions. This makes the partial vapor pressure of the vanadium comprising the volatile molecular species unstable and varying with time, and leads to spatially nonuniform vanadium doping of the grown SiC crystal (such as SiC crystal 24).
The aforementioned problem of spatially nonuniform vanadium doping was addressed in U.S. Pat. Nos. 7,608,524; 8,216,369 and US 2011/0303884, which are all incorporated herein by reference, wherein vanadium doping was accomplished by disposing the source of vanadium inside a doping capsule made of an inert material thus eliminating direct contact between the vanadium source and the SiC source 21. This doping arrangement is shown in FIG. 2.
With reference to FIG. 2, vanadium solid dopant 225 is included in an inert capsule 226 which, generally, is made of graphite. Capsule 226 includes at least one calibrated capillary 227 of predetermined diameter and length. Each capillary 227 allows controlled effusion of vanadium vapor from capsule 226. Doping capsule 226 can be placed on the surface of SiC source 221, as illustrated in FIG. 2, beneath SiC source 221 (on the bottom of growth crucible 220), or buried in the bulk of SiC source 221. FIG. 2 also shows vapor transport 223 of volatile molecular species to SiC seed 222 where the species condense on SiC seed 222 causing growth of SiC single crystal 224 on SiC seed 222.
Implementation of doping capsule 226 improved the uniformity of vanadium doping, but only for vanadium concentrations around 1·1017 atoms-cm−3 and around 1·1016 atoms-cm−3. This was due to the fact that the temperature of the solid vanadium source 225 could not be controlled independently. Accordingly, the partial pressure of vanadium inside growth crucible 220 could not be controlled independently as well. Therefore, when elemental vanadium was used in doping capsule 226 as a vanadium doping source 225, the vanadium concentration in the crystal was about 1·1017 atoms-cm−3. However, when vanadium carbide (VC) was used in doping capsule 226 as a vanadium doping source 225, the vanadium concentration in the crystal was about 1·1016 atoms-cm−3. Thus, vanadium concentrations between 1·1016 atoms-cm−3 and 1·1017 atoms-cm−3 or vanadium concentrations below 1·1016 atoms-cm−3 could be achieved reliably.
Gas-assisted PVT processes are known generally in the art. Such PVT processes include: APVT, HTCVD, HCVD, CF-PVT and M-PVT. All these modifications of SiC sublimation growth were created with the aim of achieving better crystal purity, longer growth cycle, steady-state growth, control over the vapor phase stoichiometry, and improved doping.
Advanced PVT (APVT). FIG. 3 is a schematic representation of an APVT growth cell, e.g., of the type disclosed in U.S. Pat. No. 5,985,024. In APVT growth, pure silicon 321 is included at the bottom of growth crucible 320 and melted upon heating. A gaseous carbon precursor (propane, C3H8) is introduced via a gas conduit 331. This carbon-bearing gas precursor 331 reacts with silicon vapor emanating from the molten silicon 321. The products of reaction migrate towards SiC seed 322 and precipitate on it causing growth of SiC single crystal 324 on SiC seed 322. Gaseous byproducts leave the crucible through open passages 333. FIG. 3 also shows chamber 31 (similar to chamber 1) and RF coil 311 (similar to RF coil 11).
High Temperature CVD (HTCVD). FIG. 4 is a schematic diagram of a HTCVD SiC growth cell. Details regarding HTCVD growth can be found in M. B. J. Wijesundara and R. Azevedo, “Silicon Carbide Microsystems for Harsh Environments”, Chapter 2: SiC Materials and Processing Technology, pp. 40-44. Springer Science and Business Media, LLC 2011, EP 0835336 and EP 0859879. Silicon and carbon gaseous precursor gases, namely, silane and propane, respectively, are input into crucible 420 via coaxial inlets 431 and 432. Once inside crucible 420, silane undergoes thermal dissociation leading to the formation of Si clusters. These Si clusters react with the carbon precursor gas and form SixCy clusters. Driven by a vertical vapor transport 423, the SixCy clusters enter a higher-temperature zone, where they, in similarity to the conventional PVT, sublimate to form Si and C including vapor species, such as Si, SiC2 and Si2C. These species migrate towards SiC seed 422 and precipitate on SiC seed 422 causing growth of SiC single crystal 424 on SiC seed 422. Gaseous byproducts leave the crucible through open passages 433. FIG. 4 also shows chamber 41 (similar to chamber 1) and RF coil 411 (similar to RF coil 11).
Halide CVD (HCVD). A HCVD growth cell is shown schematically in FIG. 5. Details regarding the HCVD growth can be found in A. Polyakov et al. “Halide-CVD Growth of Bulk SiC Crystals”, J. Mat. Sci. Forum (2006) Vol. 527-529, 21-26. Fanton et al. US 2005/0255245 “Method and Apparatus for the Chemical Vapor Deposition of Materials”. The HCVD growth process is similar to the HTCVD growth process, with the exception of different chemical reactions involved due to the presence of halogen (chlorine) in the system. A chlorinated silicon precursor (SiCl4 diluted by Ar) and a carbon precursor (C3H8 or CH4 diluted by H2/Ar) are supplied upward into crucible 520 via coaxial inlets 531 and 532, respectively. At high temperatures and while still inside inlets 531 and 532, these precursors dissociate yielding gaseous molecules of SiCl2 and C2H2. In a mixing zone 581, which is situated near SiC seed 522, SiCl4, SiCl2, C2H2 and H2 react in the gas phase according to the following summary equation (written without stoichiometric coefficients):SiCl2(g)+SiCl4(g)+C2H2(g)+H2(g)SiC(s)+SiCl(g)+HCl(g)The net effect of the above reaction is precipitation of solid SiC on SiC seed 522 and growth of a SiC single crystal 524 on SiC seed 522. Gaseous byproducts (HCl, SiCl) and carrier gases (Ar, H2) leave crucible 520 through the open bottom passages 533. FIG. 5 also shows crucible 51 (similar to crucible 1) and RF coil 511 (similar to RF coil 11).
Continuous Feed PVT (CF-PVT). A CF-PVT growth cell is shown in FIG. 6. Details regarding CF-PVT growth can be found in D. Chaussende et al. “Continuous Feed Physical Vapor Transport Toward High Purity and Long Boule Growth of SiC”. J. Electrochem. Soc. 2003, Vol. 150, issue 10, G653-G657. The method of CF-PVT growth is a hybrid between the PVT and HTCVD growth processes. The CF-PVT growth cell is divided into two zones: PVT zone 6101 and HTCVD zone 6102, said zones separated by graphite foam 635 which supports SiC source 621. Tetramethylsilane (TMS) 634 including both silicon and carbon is used as a single gaseous SiC precursor. TMS 634 is input into crucible 610 via inlet 638 by a flow of argon carrier gas. In order to dilute and remove reaction products from the growth cell, pure argon is supplied through lateral inlets 637. Thermal dissociation of TMS 634 occurs in HTCVD zone 6102 and leads to the formation of microscopic SiC clusters. These SiC clusters are transported by the argon flow to a higher-temperature sublimation zone where they vaporize. These vapors diffuse through porous graphite foam 635 and feed solid SiC source 621 disposed on foam 635. The solid SiC source 621 vaporizes leading to the growth of SiC single crystal 624 on SiC seed 622. Gaseous byproducts from the HTCVD zone 6102 leave crucible 610 through open passages 633. FIG. 6 also shows chamber 61 (similar to chamber 1) and RF coil 611 (similar to RF coil 11).
Modified PVT Method (M-PVT). A M-PVT cell is shown in FIG. 7A. Details regarding M-PVT growth can be found in R. Muller et al., “Growth of SiC Bulk Crystals with a Modified PVT Technique”, Chem. Vap. Deposition (2006), 12, 557-561. In essence, the M-PVT growth method is a PVT process with the added capability of delivering small amounts of Si and/or C gaseous precursors and/or dopants into a growth crucible 720 via a gas conduit 731. The M-PVT growth method has been used for the growth of aluminum-doped SiC crystals. See T. L. Straubinger et al. “Aluminum p-type doping of SiC crystals using a modified physical vapor transport growth method”. J. Cryst. Growth 240 (2002) 117-123. In one embodiment of M-PVT, trimethylaluminum (TMA) is used as a gaseous Al precursor supplied via a gas conduit 731. In another embodiment of M-PVT shown in FIG. 7B, elemental aluminum 790 is included in an external reservoir 791 connected to gas conduit 731 (FIG. 7A). The temperature of reservoir 791 is controlled by placing it at a pre-determined distance from growth crucible 720. The temperature of reservoir 791 is sufficiently high to melt aluminum and generate aluminum (Al) vapors 792 which are delivered into the growth crucible 720 with the flow of argon. FIG. 7A also shows chamber 71 (similar to chamber 1) and RF coil 711 (similar to RF coil 11).
The above-cited prior art gas-assisted PVT techniques had potential advantages, such as superior purity and stoichiometry control, but also had limitations and drawbacks. In the cases of APVT, HTCVD, HCVD and CF-PVT growth, the drawback is the open nature of the growth crucible. In all of the aforementioned processes, the presence of open passages leads to severe losses of vapors and gases and to very low crystallization efficiency. In the case of M-PVT (FIG. 7A), the drawback is interference by the gas flow or vertical vapor transport 723 coming from gas conduit 731 with the growth of SiC single crystal 724. These and other drawbacks avoided these techniques from becoming viable commercial competitors to standard PVT sublimation growth.
Vanadium doping of SiC using vanadium gaseous precursors has been explored in 4H—SiC CVD epitaxy. Ferrocene-type vanadium metalorganic compounds have been used in CVD SiC epitaxy carried out at 1370-1440° C. See H. Song et al., “Homoepitaxial Growth of Vanadium-Doped Semi-Insulating 4H—SiC Using Bis-Trimethylsilymethane and Bis-Cyclopentadienylvanadium Precursors”. J. Electrochem. Soc. 155 (2008) p. H11-H16. The ferrocene bath (bubbler) was maintained at temperatures between 50° C. and 110° C., and H2 was used as a carrier gas flowing at a rate of 10 sccm. In the epilayers grown at 1440° C., the maximum achieved resistivity was about 107 Ohm-cm. In the epilayers grown at 1370° C., higher resistivity values were observed, but the epilayer quality was poor.
Organometallic vanadium precursors were used by B. Landini et al. in CVD growth of semi-insulating SiC epilayers. See Landini et al., “CVD Growth of Semi-Insulating 4H—SiC Epitaxial Layers by Vanadium Doping”. Abstracts of 39th Electronic Materials Conference, Jun. 25-27, 1997, Fort Collins, Colo. Landini et al., “Vanadium Precursors for Semi-Insulating SiC Epilayers”, 1998 DoD-MDA SBIR/STTR Phase I Award ID: 41218. Landini et al., U.S. Pat. Nos. 6,329,088 and 6,641,938. The growth temperatures were, between 1200° C. and 1700° C. No details are available on the composition of the precursors, resistivity and quality of the produced SiC epilayers.
Generally, vanadium organometallic compounds dissociate at relatively low temperatures, typically, between 200 and 300° C., leading to precipitation of solid vanadium carbide(s). Such precipitation can occur even before the precursor is delivered into the heated SiC growth (reaction) zone.
Vanadium tetrachloride (VCl4) as a precursor in CVD chloro-carbon epitaxy was explored in B. Krishnan et al., “Vanadium Doping Using VCl4 Source during the Chloro-Carbon Epitaxial Growth of 4H—SiC”. J. Cryst. Growth, 321 (2011) pp. 8-14. The goal was to produce strongly compensated 4H—SiC epilayers. CVD growth was performed in a hot-wall CVD reactor at 1450° C. and 1600° C. with H2 as a carrier gas. CH3Cl and SiCl4 were used as chlorinated carbon and silicon precursors, respectively. Delivery of VCl4 into the growth zone was achieved by bubbling H2 through liquid VCl4 maintained at 20° C. Depending on the H2 flow rate, the vanadium concentration in the epilayers was between 1·1016 atoms-cm−3 and (2-3)·1017 atoms-cm−3. The highest resistivity observed was about 5·105 Ohm-cm.
It is believed that sublimation growth of vanadium-doped, bulk SiC single crystals using a gaseous vanadium source (precursor) injected into the growth cell during growth is not known in the art or obvious in view of the prior art.